Optical devices designed to allow to photograph any page of a book while opening the book by an angle as small as possible have been proposed before.
An example of such a device, allowing photographing of both pages of a book at a time, is described for example in German Patent DE 35 46 404. In this patent, a number of implementations of transparent plates combined with mirrors are described. In some implementations, the number of mirrors used is as high as nine. The German Patent does not cite portability and ease of use as goals of the invention. There is also no indication on how to aim the camera at the center of book pages of variable sizes.